Acronym for Portable Operating System Interface UniX.
POSIX is a large (some might even say unwieldy) standard (officially IEEE Standard 1003) specifying things such as the behaviour (arguments, etc) of standard programs and utilities that must exist on a system and C library functions. The theory is that if you write a program that conforms to POSIX standards, it will compile and run on any POSIX-compliant operating system.
The current standard is available at: http://www.opengroup.org/onlinepubs/009695399/ but is not for re-distribution. There is also special permission for extracts from the (copyrighted) standard to be used in Linux manual pages.
It used to be significantly harder for individuals to get copies of the standard, leading to comments such as
/* * ok, as I hadn't got any other source of information about * possible error numbers, I was forced to use the same numbers * as minix. * Hopefully these are posix or something. I wouldn't know (and posix * isn't telling me - they want $$$ for their f***ing standard). ... */
in errno.h from the Linux kernel (version 0.01).
1003.1 defines the core functions and behaviours, as well as things like the standard ErrorMessages.
1003.1b includes real-time extensions.
1003.2 defines extra things such as standard RegularExpression syntax and behaviour, asynchronous I/O, and real time signal queues.
See also POSIXLY_CORRECT.
Standardisation of the the pronounciation of the standards name I think is perhaps taking things too far.
POSIX® is a registered trademark of the Institute of Electrical and Electronic Engineers, Inc.
As an example what it was like trying to standardise the different functionality of the different unices, consider this from the documentation for gettext(1):
There are two competing methods for language independent messages: the X/Open `catgets' method, and the Uniforum `gettext' method. The `catgets' method indexes messages by integers; the `gettext' method indexes them by their English translations. The `catgets' method has been around longer and is supported by more vendors. The `gettext' method is supported by Sun, and it has been heard that the COSE multi-vendor initiative is supporting it. Neither method is a POSIX standard; the POSIX.1 committee had a lot of disagreement in this area. Neither one is in the POSIX standard. There was much disagreement in the POSIX.1 committee about using the `gettext' routines vs. `catgets' (XPG). In the end the committee couldn't agree on anything, so no messaging system was included as part of the standard. I believe the informative annex of the standard includes the XPG3 messaging interfaces, "...as an example of a messaging system that has been implemented..."
POSIX compliant systems include: